Potential of La-Doped SrTiO3 Thin Films Grown by Metal–Organic Vapor Phase Epitaxy for Thermoelectric Applications

La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal–organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal–organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal–organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden–Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications.


Post-annealing
As-grown SrTiO 3 thin films were annealed in pure oxygen flow at 800°C for 1 h. Figure S1 reveals that both position and shape of the film contribution have not significantly changed after post-annealing, no additional peak is observed, only thickness oscillations are slightly more pronounced.Therefore, we conclude that the films do not contain a significant amount of oxygen vacancies.(marked by an asterisk) corresponds to the substrate peak caused by the Cu K line.

Coherent film growth
Reciprocal space maps (RSM) have been recorded for the undoped as well as La-doped SrTiO 3 thin films.Since the vertical lattice parameter of the films is only slightly larger than that of the SrTiO 3 substrate (see Fig. S1b), film and substrate contributions appear almost at the same q z value.This is shown for the undoped film (Fig. S2(a)) and exemplarily for two different La concentrations (Fig. S2(b) and (c)).Also, the in-plane lattice parameters of films and the SrTiO 3 substrates match within the resolution limit which indicates coherent film growth.For the La-doped films, the background is slightly enhanced compared to the undoped film.

Evaluation of the XRD measurements
Simulations of the XRD scans were performed by the software RCRefSimW (Version 1.08) to evaluate film thickness and peak position.With the simplest approach of a homogeneous film coherently grown on a substrate no decent match between simulation and measurements curves could be achieved, because the simulation software does not take into account an elevated background due to dislocations and/or vacancies.Furthermore, it was also only possible to reproduce the position of the film Bragg reflection and the thickness fringes for the undoped film by the use of a single film component (see Fig. 3a).Whereas for the La-doped films, at least 2 -3 components were always necessary to achieve a sufficiently good fit of the measured data.However, the accuracy of the results with regard to the vertical lattice parameter (from the angular position of the film peak) and the film thickness (from the distance of the thickness oscillations) could not been significantly improved with application of more than three sublayers for the film.Therefore, the maximum number of sublayers in the simulations was three.

Electrical properties
We investigated a 30nm-thick, undoped SrTiO 3 reference film grown under similar conditions as those discussed in the manuscript as well as an as-purchased, undoped SrTiO 3 substrate by Hg-CV measurement (method and setup described in Ref. S1).This method would indicate the depletion layer thickness under which a potential carrier system exists.
The measured capacitance of 3.2 pF for both, substrate and reference layers, is largely independent of DC bias and frequency confirms the absence of mobile carriers.(This capacitance would correspond to an unphysically large depletion layer thickness of ~100 µm assuming a relative dielectric constant of 200 for SrTiO 3 ).Current-voltage measurements -10V to 10V of the reference layer using the same Hg-probe also indicated insulating behavior (R > 10 12 ).Thus, we can safely exclude a parallel conductivity in the SrTiO 3 substrate under the discussed La-doped SrTiO 3 layers.The absence of a carrier system in the substrate is also in agreement with the less reducing growth conditions in our MOVPE (T growth = 710°C, p(O 2 ) = 15 mbar) than those that lead to substrate reduction in Ref. [S2] with T growth = 750°C, p(O 2 ) < 10 -3 mbar.

Figure S2 :
Figure S2: Reciprocal space maps in the vicinity of the (204) SrTiO 3 Bragg reflection of (a)